Re: [PATCH v2 1/3] arch/arm64: Introduce a capability to tell whether 16-bit VMID is available

From: Will Deacon
Date: Wed Jul 21 2021 - 11:23:26 EST

On Wed, Jun 16, 2021 at 04:56:04PM +0100, Shameer Kolothum wrote:
> From: Julien Grall <julien.grall@xxxxxxx>
> At the moment, the function kvm_get_vmid_bits() is looking up for the
> sanitized value of ID_AA64MMFR1_EL1 and extract the information
> regarding the number of VMID bits supported.
> This is fine as the function is mainly used during VMID roll-over. New
> use in a follow-up patch will require the function to be called a every
> context switch so we want the function to be more efficient.
> A new capability is introduced to tell whether 16-bit VMID is
> available.

I don't really buy this rationale. The VMID allocator introduced later on
caches this value in the static 'vmid_bits' variable, and that gets used
on vCPU enter via vmid_gen_match() in the kvm_arm_update_vmid() fastpath.

So I would prefer that we just expose an accessor for that than introduce
a static key and new cpufeature just for kvm_get_vttbr().